The invention resides in a method for increasing the throughput of packages in rotary kiln waste material combustion plants which are generally tubular chambers rotating about an axis of symmetry (motor driven rotary tube). At one end, the rotating tube opens into a post combustion chamber leading to an exhaust gas channel and at the other end fuel is supplied by burners, nozzles and solid material transport devices. By way of the solid material transport devices, packages of (liquid high caloric) waste material is discontinuously added and burnt in the rotary kiln. Rotary kilns are particularly used for the combustion of heterogeneous combustible materials such as industrial waste and particularly waste materials, which need to be monitored.
The gas phase combustion process area of a combustion plant is determined essentially by conditions such as residence time, temperature and mixing as well as stoichiometry. Without optimizing the combustion process by these values, already in the combustion space strands of excess air flows as well as areas with local air deficiencies can form so that the oxygen content varies highly locally and also with time. The mixing (turbulence) influences herein mainly the formation of local strands, the transient combustion in connection with packages because of the stoichiometry (O2 supply) the formation of time-variable strands. Both ways of forming strands lead to a non-uniform and incomplete combustion in the combustion space and result in the emission of noxious materials (CO). Particularly the CO content serves as an indicator for the combustion quality.
The formation of time-variable strands in the combustion space is particularly problematic in connection with the combustion of packages in rotary kilns since the packages are supplied discontinuously. When a package is supplied by the transport device to the combustion space of the rotary kiln, the package opens up more or less suddenly—depending on the calorie content. With the thermal conversion of the suddenly released high-caloric content of a package, the thermal rotary kiln loading is suddenly highly increased and the available oxygen amount is locally much reduced.
But also other exhaust gas species concentrations such as moisture (H2O), carbon dioxide (CO2) or carbon monoxide (CO) change suddenly with the combustion of packaged material. As a result, because of the combustion-based oxygen consumption, also substantial amounts of unburned hydrocarbons, soot and particularly CO (as concentration peaks) are formed in the rotary kiln, which cannot be fully eliminated in the post combustion chamber even with the use of burners. Subsequently, the noxious compounds pass through the plant including the exhaust gas cleaning equipment almost uninhibitedly and are discharged via the chimney into the atmosphere.
Since all waste combustion plants are subjected to tight emission limits, the CO-concentration at the exhaust duct is, based on the half hour average or, respectively, the day average, the limiting factor for the combustion of packages in the rotary kiln (half hour average value: 100 mg/Nm3 CO, day average value: 50 mg/Nm3 in accordance with BImSchV).
It is known that, for reducing the CO formation during the combustion of packages, highly over-stoichiometric air amounts are supplied to the rotary kiln, in order to accommodate the fuel release peaks in the form of soot, organic C and CO (influencing the stoichiometry by increasing the combustion air amount). Since the exhaust gas volume flow is normally capacity limiting the waste flow is substantially reduced by this procedure. The excess air flow which is highly over-stoichiometric and has a cooling effect in the kiln additionally results in lower combustion temperatures and consequently to a deterioration of the reaction conditions in the combustion space.
It is also known to influence the stoichiometry of the combustion of packaged waste by the addition of oxygen enriched combustion air or the addition of oxygen by way of separate nozzles in such a way that an increased flow of waste in the form of packages is possible. By substituting combustion air by oxygen-enriched air or, respectively, by the addition of oxygen to the combustion process, first the stoichiometry (O2 supply) is substantially increased, while the temperature and exhaust gas volume flow remain essentially constant.
With increased supply of high caloric packages, the overall stoichiometry (O2 supply) drops again whereas the exhaust gas volume remains essentially constant. With the increase of the oxygen content in the combustion air, the combustion temperature in the rotary kiln is increased while the exhaust gas volume remains the same, since the amount of ballast air (air nitrogen) is reduced and must not be heated to the combustion temperature. An increased combustion temperature again leads to an increased temperature load in the combustion chamber of the rotary kiln (melting of the slack deposits). Another disadvantage with the use of oxygen-enriched combustion air or additional oxygen injection into the combustion chamber is the economic viability resulting from the increase in expenses by the oxygen enrichment and the safety considerations.
A separate control of the fuel-air ratio of individual gas and oil burners on the basis of signals of optical sensors is also known.
DE 100 55 832 A1 discloses such a control of the fuel-combustion air-mixture of oil and gas burners on the basis of photo sensors which monitor optically the flame radiation.
DE 197 46 786 C2 further discloses an optical flame monitor with two semiconductor detectors for oil and gas burners for the monitoring of the flames and the control of the fuel-air ratio or, respectively, the fuel supply, wherein the spectral distribution of the flame radiation is used as the input signal for the control.
DE 196 50 972 C2 also includes such a control for monitoring and controlling the combustion process by measuring the radiation by sensor-based detection of a narrow—as well as wide-band spectral range of a flame. The purpose is to maintain of high combustion efficiency and, at the same time minimal toxic emissions.
The cited state of the art however comprises only solutions for particular single problems with respect to the adjustment of individual oil or gas burners and not for the control of the overall process of a combustion plant (rotary kiln).
In order to achieve a substantial improvement in the plant efficiency by optimizing the rotary kiln/post combustion operating procedure, a rapid (and simultaneous) determination of the values defining the combustion procedure in the rotary kiln (CO, soot, O2, CO2, or H2O) is necessary. Conventional sensors, or, respectively, sampling procedures, wherein exhaust gas is drawn from the process result in long response times.
These monitoring procedures are not suitable to determine the incomplete combustion (for example, by way of concentration changes of individual species such as soot, CO, O2, H2O or CO2) in the rotary kiln sufficiently rapidly. For a rapid control of the combustion process, an in-situ determination of the combustion-relevant species such as O2, CO2, H2O, CO or soot (optical measurement procedures) in the combustion space with short response times (tAntwort<<tReaction) and high selectivity is necessary. If the detection of these components is too slow, the products of an incomplete combustion cannot be fully decomposed in the rotary kiln by appropriate procedures. The speed with which the concentration peaks move through the plant and the corresponding necessary reaction time of the control process depend on the plant material flow.
It is the object of the present invention to increase the processing capacity for high caloric packages in rotary kilns of the type referred to above while maintaining emission limits without the limitations described above.